1. Field of the Invention
The present invention relates to a lean burn internal combustion engine, and in particular, relates to a control of the temperature of a lean sensor used in such an engine for a detection of the air-fuel ratio therein.
2. Description of Related Art
Known in a prior art is a lean burn combustion engine having an operating area, for example, a low load condition, wherein a lean combustible mixture is introduced into the engine to increase the fuel consumption efficiency. In the lean burn engine, a basic fuel injection amount, which is an amount of fuel needed for providing a theoretical air-fuel ratio at a combination of an engine speed and an engine load parameter such as the intake pressure, is first calculated and then, to obtain the lean air-fuel mixture, a lean correction factor having a value smaller than 1.0 is multiplied by the basic fuel injection amount. A lean correction factor map is provided and is constructed by values of the lean correction factor with respect to combinations of the engine speed and the intake pressure. When the engine enters a power area from the lean area, due to a depression of an accelerator pedal, a fuel enrichment correction is carried out to obtain a desired engine torque.
To obtain a desired air-fuel ratio, a lean sensor is provide in the exhaust pipe of the engine. This lean sensor comprises a solid electrolyte body, such as zirconia, having opposite surfaces on which electrodes are formed, and a diffusion velocity control layer formed on one of the electrodes and in contact with the exhaust gas to be detected. A voltage control means is provided for obtaining a predetermined voltage across the electrodes, to obtain a pumping electric current for generating a flow of oxygen ions from the exhaust gas to be detected, via the speed control layer, under a diffusion condition; this pumping electric current is proportional to the air-fuel ratio.
The lean sensor is usually provided with a heater for obtaining a predetermined temperature of the body, to provide a desired output characteristic. When a constant electric current is applied to the heater, the temperature will change in accordance with that of the exhaust gas, which varies in accordance with the engine speed and engine torque. Therefore, to compensate for this change in the temperature of the exhaust gas, a system has been proposed wherein an electric current applied to the heater is controlled in accordance with the engine speed and an engine torque parameter such as the intake pressure, to obtain a predetermined constant temperature. To accomplish this, a map of values of the electric current applied to the heater with respect to combinations of the engine speed and the intake pressure, as an engine load parameter, is provided. A map interpolation calculation is carried out to obtain a value of the electric current applied to the heater in accordance with a detected combination of the engine speed and the intake pressure. See Japanese Unexamined Patent Publication No. 60-235046.
Nevertheless, sometimes the obtained temperature of the heater is different from the value calculated from the engine speed and the engine torque. When the engine state is changed by an opening of the throttle valve, in place of a lean air-fuel ratio map based on an intake pressure and engine speed, a second lean air-fuel ratio map is employed to obtain a less lean air-fuel mixture, to thereby obtain a desired increase in the torque, as disclosed in allowed U.S. patent application Ser. No. 528,565, filed on May 24, 1990. As a result of the employment of a less lean air-mixture, the temperature of the exhaust gas becomes higher than that obtained when the lean air-fuel mixture is employed, and accordingly, sometimes the temperature of the sensor element is excessively increased when the heater current is calculated from the basic map based on the intake pressure.